Mast cell tumors (MCT) are the most common malignant skin tumors in dogs. Surgery is the primary recommended treatment, and post-surgical follow-up is crucial for detecting recurrences and disease progression. While surgery tends to be curative in most cases, minimal residual disease (MRD) refers to malignant cells, undetectable by conventional methods, that remain after surgery, increasing the risk of relapse. This study explores whether circulating tumor DNA (ctDNA), small pieces of genetic material that can shed from the tumors to the blood, can be used for monitoring MRD. CtDNA is capable of reflecting tumor burden in real time and can be used to identify specific cancer mutations. This study aims to analyze ctDNA to identify mutations of interest and serially quantify it to detect MRD in dogs with MCT after surgery.